The majority of styrene monomer produced commercially is prepared by the dehydrogenation of ethylbenzene in a vapor phase, fixed catalyst-bed reactor. Each pass through the reactor converts about 60 to 75% of the ethylbenzene feed to styrene. The dehydrogenation reaction of ethylbenzene yields a crude styrene stream which is a mixture containing substantial portions of styrene and ethylbenzene as well as smaller amounts of reaction by-products and impurities, such as, benzene, toluene, alpha-methyl styrene (AMS) and heavies. To produce saleable styrene monomer product, the non-styrene components must be separated from the styrene product via distillation. Styrene monomer must be purified to a concentration of at least 99% by weight before it can be used by downstream polymer processes. The main components to be separated from styrene in the purification process are benzene, toluene, un-reacted ethylbenzene, AMS and heavies that are referred to as residue (i.e., compounds that are in the C10 to C141 range).
The current practice in the industry is to use a minimum of three distillation columns to obtain the aforementioned level of styrene product purity. The function of the distillation columns is to recover the benzene/toluene byproduct, the unreacted ethylbenzene, and separate AMS and heavier residue compounds from the styrene product. Overall the distillation process requires a large amount of energy (steam) to purify the styrene product.
Moreover, the separation by distillation of the styrene monomer (SM) from the unreacted ethylbenzene (EB) presents a considerably difficult problem due primarily to their close similarity in volatility. In this regard, the boiling points of ethylbenzene and styrene are within approximately 10° C. of each other at 760 mm Hg, which makes separation by fractional distillation difficult and costly. Conventionally, this EB/SM separation has been accomplished by distillation under vacuum conditions in large, sophisticated, and expensive distillation columns due to the large number of theoretical plates required to effect a good separation. In the conventional distillation process, unreacted ethylbenzene from the dehydrogenation reaction section is separated from styrene in a single distillation column. In the typical design, a large number of theoretical stages (between 85 and 115) are required to effect the required separation in an economically effective manner. This single unit operation accounts for between 70 and 80 percent of the total distillation section heat input. In a typical styrene manufacture process plant, the separation of unreacted ethylbenzene from styrene product accounts for approximately 20-30% of the entire plant's steam consumption.
Additional problems associated with the distillation of the styrene monomer are related to the styrene monomer's inherent reactivity. Because the styrene monomer polymerizes even at ambient temperatures forming insoluble solids, this undesirable reactivity makes distillation of the styrene monomer challenging. Since the rate of styrene polymerization increases with increase in temperature, conventional practice involves operating the distillation columns of commercial styrene plants at low pressures to reduce boiling temperatures and thereby reduce the extent of adverse polymerization.
As a result of these various process difficulties, and in particular, the associated large energy consumption requirements and costs, considerable incentive has existed for many years to develop alternative means of effecting this separation process, which is more viable from economic and ease of operation standpoints. A number of patents have attempted to address these problems in a variety of ways.
U.S. Pat. No. 6,171,449 teaches methods of energy consumption improvements by recovering at least a portion of the heat contained in an EB/SM splitter overhead stream via use of a cascade reboiler scheme in which the separation of ethylbenzene and styrene is carried out in two parallel distillation columns operating at different pressures, with the overhead of the high pressure column providing the heat required to reboil the low pressure column. U.S. Pat. No. 4,628,136 teaches a method of recovering the heat contained in the overhead of the EB/SM splitter by using this stream to boil an azeotropic mixture of ethylbenzene and water, which, once vaporized, is subsequently transferred to the reaction system where dehydrogenation of ethylbenzene to styrene takes place. The method described in the U.S. Pat. No. 4,628,136, however, requires that the EB/SM splitter operate at a pressure that is sufficiently high as to enable the transfer of the azeotropic mixture of ethylbenzene and water vapor into the reactor system without the use of a compressor. This patent also specifies that the temperature difference between the condensing EB/SM splitter overhead and the boiling azeotropic mixture of ethylbenzene and water should be in the range of between and 2 and 10° C. Given this temperature constraint, one can derive a relationship between the pressure at which the azeotropic vaporization is taking place and the required overhead pressure of the EB/SM splitter.
For economic reasons it is desirable to lower the amount of steam (i.e., energy) requirements in the process to purify styrene monomer. Thus, in view of the above, it would be beneficial to have a method of reducing the steam usage while also having the ability to independently add heat into the process.